1. Field of the Invention
The present invention relates to an electronic endoscope, and more particularly to an improvement of a white balance adjustment in an electronic endoscope when a color still image is picked up.
2. Description of the Background Art
In a conventional endoscope, when a moving image pickup operation is carried out, continuous lighting of a light source is achieved by applying a DC current, and, in turn, when a still image pickup operation is carried out in response to a freezing command, a pulsed lighting is used by applying a modulated pulse signal. In this case, even when a change between the moving and still image pickup operations is conducted, there is no difference in brightness to obtain a clear color image on a display without incurring blur in the still image pickup operation.
Since the inside of internal organs such as stomach or the like is reddish, when it is observed by using an endoscope without having a color adjustment of picture image signals in consideration of such a reddish color, the whole of the obtained color image becomes reddish. In the conventional endoscope, in order to prevent this problem, a so-called white balance adjustment is carried out. That is, factors for making the intensity of the picture image signals for three primary colors such as red (R), green (G) and blue (B) equal when the picture image signals are picked up by imaging a standard white color plate, are obtained and multiplied to the picture image signals for the primary colors.
However, spectral characteristics such as intensity of an emitted light of a xenon lamp usually used as a light source is different between the continuous DC current lighting in the moving image pickup operation and the pulsed lighting in the still image pickup operation, as shown by a solid line A (continuous DC light) and a broken line B (pulsed light) in FIG. 5. Hence, when the operation is changed from the moving image pickup to the still image pickup, the color tone of the obtained image is changed. That is, in a conventional endoscope of this kind, the white balance adjustment is carried out without taking consideration of the intensity difference of the emitted light of the xenon lamp between the DC lighting and the pulsed lighting, and hence, when the operation is changed from the moving image pickup to the still image pickup, the color tone of the reproduced color picture image is changed. This makes it difficult for an operator to observe the inside of an internal organ and diagnosis a patient.